The development of non-petroleum sources of fuels is important in long term energy sustainability for most countries. One of these potential sources is from growing biomass and directly or indirectly using the fuel value. A criticism of this approach is that productive farm land may be displaced for these fuel crops. A less controversial source of energy is from wastes produced from agriculture. These materials include animal wastes, food wastes and crop residues. These materials may be used to produce methane using anaerobic digestion technology. Methane is the primary component of Natural Gas and may be substituted for Natural Gas in many applications. Methane when pure is a very clean burning fuel and can be used in vehicles, for heating and when used as the fuel for an engine in an electrical generator can produce electricity. Although there are many potential sources of wastes that could be anaerobically digested, most are not utilized at the present time because of unfavorable economic payback for small-scale systems. One of the significant issues is that the methane produced from anaerobic digestion is not pure but contains substantial amounts of carbon dioxide, an inert gas that dilutes the fuel value of the biogas, and contains contaminants such as sulfur compounds which have an unpleasant odor and cause significant corrosion and environmental emissions. Removal of these sulfur compounds is thus an important step in utilizing biogas.
Many techniques are known for sulfur compound removal from gases. Reactions of the compounds with iron or zinc oxides with or without catalysts, are commonly used in large scale systems where disposal of the reacted products is not a significant issue or cost. Membrane technology is also used in large systems where there is sufficient expertise on staff to maintain the operation. For smaller scale systems water scrubbing and carbon absorption beds have been used. These systems are relatively easy to operate but also produce wastes which require disposal. These wastes typically have significant odor from hydrogen sulfide, a common sulfur compound produced during anaerobic digestion. Hydrogen sulfide is a toxic gas as well as being odorous which complicates the waste disposal.
The production of charcoal from biomaterials is known. The focus was on producing a relatively clean burning solid, a fuel still used in barbecues and cooking stoves. The non-carbon elements are removed by heating the biomaterial in the absence of air and causing the volatilization of these species. More recently, there has been emphasis on converting as much of the biomaterial as possible to volatiles that have fuel value in a process known as pyrolysis. The residual carbon in both cases is somewhat porous but does not have high surface area. For high surface area carbon absorbents, chemical treatment is applied either before pyrolysis or afterwards with a second thermal treatment. This is often followed by a water wash to remove the activating chemical or the unwanted ash.
For activated carbon production, two steps are used. The first produces the elemental carbon and the second activates the carbon. For producing activated carbons with specific absorption characteristics a third step is used in which specific catalytic chemicals are loaded onto the carbon surface. As should be clear, the cost of the activated carbon product increases with the extent of processing required for the final product.
The use of waste materials as a source for activated carbon production is not commonly discussed because the carbon content is relatively low. In an article by Chen et al. (“Physical and Chemical Properties Study of the Activated Carbon made from Sewage Sludge 2002, Waste Management, 22, 755-760) they describe the use of sewage sludge; an anaerobically digested residue from municipal waste water treatment. Zinc chloride was required as an activating agent to enhance the surface area of the carbon product and the carbon content of the activated carbon was only 38.9% C by weight.
The use of anaerobic sludge was also reported to be poor for activated carbon production by Tay et al. In a publication entitled “A Comparative Study of Anaerobically Digested and Undigested Sewage Sludges in Preparation of Activated Carbons” Chemosphere 2001, 44, 53-57, Tay et al. found that the undigested sludge produced an activated carbon with higher carbon content, lower ash content, higher surface area and better phenol adsorption characteristics. Thus the use of digested sludge is discouraged.
Martin et al. (“Feasibility of Activated Carbon Production from Biological Sludge by Chemical Activation with ZnCl2 and H2SO4”, Environmental Science and Technology 1996, 17, 667-672) showed that optimal activation conditions produced a surface area of 257 m2/g with an ash content of 38% and a yield of 34%. This material had a contaminant removal of only 20% of that for a commercial activated carbon. Again this suggests that anaerobic sludge is a poor material for activated carbon production.
In a patent on activated carbons from animal manures (U.S. Pat. No. 7,524,795 issued to Lima et al.) a process is described whereby carbonization of poultry manure followed by activation is used to produce an activated carbon with good metal ion adsorption characteristics. In this work, the starting material is raw poultry manure and not the anaerobic sludge digestate from poultry manure.
A patent by Freel et al. (U.S. Pat. No. 7,199,080) describes how agricultural corn derivatives can be used to produce activated corn carbon. The process involves pyrolysing the corn derivatives to generate carbon char and then activating the char using a steam activation stage. An additional acid washing step is also suggested to remove ash from the product. This process differs from the current process by being a 2 step operation and by starting with raw corn derivatives and not the anaerobically digested solids.
Bandosz et al. (U.S. Pat. No. 6,962,616) teach a process for removing acidic sulfur species from wet gas streams using a carbon absorption material which is produced from dewatered and dried sewage sludge. This process for sewage sludge preparation requires a drying operation, addition of mineral oil and a pyrolyzing step before the carbon material is ready for use as a adsorbent. It will be clear that the current process has advantages over the process of Bandosz et al. in that no mineral oil is required and the starting material is from anaerobic digestate solids and not restricted to sewage sludges. When used for removing acidic sulfur species from a gas stream Bandosz et al. rely on inorganic constituents from the pyrolyzed solids to convert hydrogen sulfide. In claim 3 the authors state that the hydrogen sulfide reacts with the inorganic constituents of the carbon to produce sulfur dioxide or elemental sulfur or salt forms thereof. In the current process for removing hydrogen sulfide from the gas stream there is insufficient inorganic content or chemicals to oxidize the hydrogen sulfide in the gas stream. For the current process a small percentage of oxygen or air should be added to the gas stream and is the oxidant which converts hydrogen sulfide to either sulfur or solid oxidized sulfur compounds. The use of a low percentage of air addition to the gas stream to be cleaned is particularly advantageous in maximizing the capacity of our carbon material for hydrogen sulfide removal. Without this addition, hydrogen sulfide is simply adsorbed and would be a problem for the used absorbent carbon disposal. This is also a problem for commercial activated carbons which can adsorb hydrogen sulfide but are not able to convert hydrogen sulfide to elemental sulfur or oxidized sulfur salts.
The carbon material produced by the current process has the capability of converting essentially all of the hydrogen sulfide in the gas stream to sulfur or oxidized sulfur solids when the gas stream has a few percent air addition. The conversion of hydrogen sulfide to elemental sulfur has a significant advantage in that the used absorbent carbon solids can then be utilized as a secondary value-added product in horticulture or agriculture.
A patent by Beckler and Miller (U.S. Pat. No. 6,277,780) describes preparation of an activated carbon from a variety of carbonaceous sources and a thermal treatment, but in order to make this material suitable as an absorbent the material must be loaded with a phosphorous containing chemical. Clearly simple heat treatment is not sufficient for preparation of a carbon absorbent.
A patent by Khalili et al. (U.S. Pat. No. 6,030,922) describes the preparation of an activated carbon from raw sludge but which requires chemically activating the dried sludge material before thermal treatment. It is clear that this chemical addition is required to produce a useful activated carbon.
A patent by Piskorz et al. (U.S. Pat. No. 5,853,548) describes a two stage process for converting biosolids into fuels and for further heat treating the residual char to produce an activated carbon. In the first stage a temperature range of 390 to 450° C. was used. In the current process, the minimum temperature that was found to be effective was 500° C. A significant difference is that the Piskorz et al. patent has focused on using shredded biomass material rather than anaerobic digestate that is used in the current process. Microbial degradation of biomass in anaerobic digesters reduces the content of readily degradable organic species and hence would reduce the fuel value of the material. The second stage char treatment in the Piskorz et al. patent attempts to minimize the char volume by heating at a temperature of 700° C. for thermal annealing. In the current process, production of an absorbent carbon is the intended product and its minimization is not a desirable goal.
In a patent by Abe et al. (U.S. Pat. No. 5,338,462), a method for decomposing water contaminants such as hydrogen peroxide, hydrazines, quaternary ammonium salts, organic acids and sulfur-containing compounds using an activated carbon is described. The hydrocarbon source material is a nitrogen-rich protein-containing sludge or a waste material comprising microbial proteins or biologically activated sludge. This material is first carbonized at a temperature of from 150° C. to 600° C. then activated at a temperature of from 700° C. to 1,100° C. in an inert gas or a reducing gas atmosphere consisting essentially of steam or gaseous carbon dioxide.
It should be clear that the carbon sources and the end use applications are very different in the Abe et al. patent compared to the current application. The current process claims as a starting material acidogenic digestate which is the fibrous undigested cellulosic and lignin structures from anaerobic digestion of waste materials and not the biologically activated sludge material in the patent by Abe et al. The nitrogen content of our starting material would be poor for the purposes claimed in the patent by Abe et al. and the microbial proteins required by the patent by Abe et al. by would not have the structure required for producing our carbon absorbent.
In a patent by Lewis (U.S. Pat. No. 4,122,036), a process for pyrolysing sewage sludge to produce activated carbon is described. The sewage sludge, which has a high moisture content is mixed with recycled hot char from a rotating kiln to produce a dry, free-flowing product which is fed into the kiln. It is clear that this patent does not teach the conditions required by the current technology for producing a carbon absorbent from anaerobic digestate.
Thus, what is needed is an improved process for producing a carbon absorbent material from anaerobic digestate.